1. Field of the Invention
The present invention relates broadly to methods of preparing metal oxide films and more particularly to an electroless method for the direct deposition of cuprous oxide.
2. Description of the Prior Art
Metal coated articles have been produced by various well-known electroless deposition techniques which involve contacting a receptive surface with a metal salt and a reducing agent to form a metal film. Autocatalytic techniques wherein the metal salt and reducing agent are present in a single solution and react upon contact with a catalytic surface are described in U.S. Pat. No. 2,532,283 and No. 2,532,284 for the deposition of cobalt and U.S. Pat. No. 2,956,900 for the deposition of nickel. Exhaustive techniques wherein the metal salt and reducing agent react rapidly upon contact with each other and so are applied to the surface to be coated in separate solutions are described in U.S. Pat. No. 3,671,291. The methods described in U.S. Pat. No. 3,671,291 involve contacting a reactive substrate with an acidic metal salt solution and an alkaline reducing solution.
Such electroless coating techniques have been successfully employed for the direct deposition of thin, transparent metallic films of iron, cobalt, nickel, copper, silver and mixtures thereof on non-metallic substrates, particularly large sheets of glass. The films so produced have suitable transmittance, reflectance and uniform thickness to provide coated articles with important solar energy control characteristics and a uniform aesthetic appearance.
In U.S. Pat. No. 3,457,138, an improved electroless method for depositing copper films on glass is disclosed. The method involves contacting an activated glass surface with an alkaline aqueous solution of a copper salt, a reducing agent, and a tartrate salt in the presence of nickel or cobalt. The pH of the solution may be from about 8 to 12, preferably from about 9 to 11. Copper film forming solutions having acidic pH are avoided since under acidic conditions the copper film formation does not proceed satisfactorily.
Electrolessly deposited metal films may be oxidized in order to obtain optical and spectral properties which differ from the properties of the initially deposited metal films. In U.S. Pat. No. 3,846,152, articles with selective reflectance and transmittance properties for radiation over an extended spectral range are disclosed. Such articles are produced by coating a substrate with a first metal film of silver, iron, cobalt, chromium or nickel; coating with a second metal film of copper or iron by electroless deposition using an alkaline aqueous solution of a metal salt and reducing agent; and heating to convert the second metal film to metal oxide. In U.S. Pat. No. 3,920,864, a method is disclosed for treating a copper-coated article with a solution which results in partial replacement of the copper by silver. The copper-silver composite film is subsequently oxidized by exposure to moist air.
As an alternative to the subsequent oxidation of electrolessly deposited metal films, a method is disclosed in U.S. Pat. No. 3,978,272 for the direct deposition of metal oxide films. Such films are deposited by spraying a solution of a metal compound onto the substrate which is sufficiently hot to thermally decompose the metal compound thus forming a metal oxide film by pyrolysis.